Hydroformylation generating linear (normal) and branched (iso) aldehydes, in which the number of carbons increases by 1, by reacting a variety of olefins with a mixture of carbon monoxide (CO) and hydrogen (H2) generally called synthesis gas and in the presence of a homogeneous organometal catalyst and a ligand was first discovered by Otto Roelen of Germany in 1938.
Hydroformylation also known as oxo synthesis is a very important reaction in connection with a homogeneous catalyst reaction. At present, approximately 12 million tons of a variety of aldehydes comprising alcohol derivatives are produced and consumed through oxo synthesis worldwide (SRI report, November 2012, 7000I page 10).
In oxo synthesis, a variety of aldehydes is oxidized or hydrogenated after condensation such as aldol and the like, thereby being transformed into a variety of acids and alcohols comprising long alkyl groups. In particular, hydrogenated alcohols of aldehydes through oxo synthesis are called oxo alcohols. Oxo alcohols are widely used in industry as solvents, additives, raw materials of a variety of plasticizers, synthetic lubricating oils, and the like.
As a catalyst of the hydroformylation reaction, activity of a metal carbonyl compound catalyst was known. Cobalt (Co) and rhodium (Rh) based catalysts are mainly used in industry. An N/I selectivity (ratio of linear (normal) to branched (iso) isomers), activity, and stability of the aldehydes depend on ligand types applied to these catalysts and driving conditions.
At present globally, 70% or more of oxo process-related plants use a low pressure oxo process, in which a large amount of a phosphine ligand is applied to a rhodium based catalyst, due to advantages such as high catalyst activity, a high N/I selectivity, and a relatively easy reaction conditions, in spite of drawbacks such as high catalyst costs, catalyst activity reduction due to poisoning, and the like.
As a central metal of an oxo catalyst, in addition to cobalt (Co) and rhodium (Rh), transition metals such as iridium (Ir), ruthenium (Ru), osmium (Os), platinum (Pt), palladium (Pd), iron (Fe), nickel (Ni), and the like may be used. However, each of the metals is known to have the following catalyst activity relationship: Rh>>Co>Ir, Ru>Os>Pt>Pd>Fe>Ni. CO, Rh, Pt, and Ru, which are Group VIII transition metals, exhibit high catalyst activity in an oxo reaction. Pt and Ru are only used in academic research. At present, rhodium and cobalt are mainly used in most industrial oxo processes. As representative examples, there are HCo(CO)4, HCo(CO)3PBu3 and HRh(CO)(PR3)3.
As ligand types used in an oxo process, there are phosphine (PR3, where R is C6H5, or n-C4H9), phosphine oxide (O═P(C6H5)3), and phosphite. When rhodium is used as a central metal, triphenylphosphine (TPP) as a ligand in connection with catalyst activity and stability is considered to be the best option. Therefore, in most oxo processes, rhodium (Rh) as a catalyst is used and TPP as a ligand is applied. In addition, it is known that TPP as a ligand is applied in an amount of 100 equivalents or more of a catalyst to improve catalyst stability.
Generally, linear aldehyde derivatives of aldehydes, products of an oxo reaction are valuable and, as such, most research on catalysts has focused on increasing a ratio of linear aldehydes.
However, there is an urgent need for technology to improve catalyst stability and reduce a use amount of ligands while improving selectivity for iso-aldehydes.